A user can own more than one communication devices, such as a mobile phone, a tablet computer, and a smart wristwatch. The tablet computer and the smart wristwatch both have communication function. Generally, each of these communication devices has a unique communication number. Moreover, the user may prefer to use different communication devices in different places and at different times, for example, to use the tablet computer when at home, to use the mobile phone at office, and to use the smart wristwatch when taking exercise.